Numerous types of equipment used for the analysis or purification of chemical compounds utilize miniature fluid conduits, such as metallic tubing, through which liquid samples pass. An analytical technique, such as liquid chromatography, uses a column (conduit) packed with a packing material in order to analyze and identify chemical properties of certain fluids. For example, an analyte may be introduced into one end of the column, and a carrier fluid then run through the column. The length of time that the analyte is retained within the column can enable analysis and identification of the analyte. A popular form of liquid chromatography is High Performance Liquid Chromatography (HPLC) in which the sample is pumped through the column under an elevated pressure, typically at 300 to 6,000 psi. Another, relatively newer liquid chromatography form is Ultrahigh Pressure Liquid Chromatography (UHPLC) in which system pressure extends upward to 1400 bar or 20,000 psi, and possibly 30,000 psi. Both HPLC and UHPLC are examples of analytical instrumentation that utilize fluid transfer at elevated pressures.
Liquid chromatography systems, such as HPLC or UHPLC systems, typically include several components. For example, such a system may include a pump; an injection valve or autosampler for injecting the analyte; a precolumn filter to remove particulate matter in the analyte solution that might clog the column; a packed bed to retain irreversibly absorbed chemical material; the HPLC column itself; and a detector that analyzes the carrier fluid as it leaves the column. These various components may typically be connected by a miniature fluid conduit, such as metallic or polymeric tubing, usually having an internal diameter of 0.003 to 0.040 inch.
All of these various components and lengths of tubing are typically interconnected by threaded fittings. Fittings for connecting various components and lengths of tubing are disclosed in prior patents and patent applications, for example, U.S. Pat. Nos. 5,525,303; 5,730,943; 5,911,954; and 6,095,572; and U.S. Patent Application Publication No. 2008/0237112, filed on Jan. 9, 2008, the disclosures of which are herein all incorporated by reference.
A typical threaded fitting 18 well known in the art is shown in FIG. 1. The threaded fitting 18 includes an internally threaded portion 20 formed near its open end that is suitable for threadably receiving a second fitting, tightening device, etc., having an external threaded portion (not shown). The fitting 18 further includes an internal passageway 24 that narrows in diameter at its distal terminus to form a female, cone-shaped chamber 28 defining a frusto-conical sealing surface 26. The cone-shaped chamber 28 is in communication with a cylindrical chamber 32 sized to receive tubing 34 therein. The cylindrical chamber 32 defines a “tube stop” 30 at its end that closely and fully receives the tip of the tubing 34.
Often, the tubing interfaces with the threaded fittings with a ferrule or similar sealing device (see ferrule 36 in FIG. 1). The ferrule includes a cone-shaped end that allows it to be compressed within the female cone-shaped chamber of the fitting and thus form a liquid-tight seal. As is well known in the art, the tubing must be seated on the bottom of the cylindrical chamber when the ferrule is received within the fitting in order to ensure good chromatography. This becomes even more critical in UHPLC where the negative effects are greater. If the tube is not bottomed out in the cylindrical chamber, the resulting chromatogram exhibits band broadening due to mixing of the sample with the mobile phase. The extra volume between the end of the tube and the cylindrical chamber bottom is known as “dead volume.” It is preferred that all fitting connections after the pump be made as “zero-dead-volume” connections to keep band broadening to a minimum. Even in connections before the pump it can be critical that there is “zero-dead-volume”because the extra volume will change the exact nature of mixing solvents, giving a different delay volume from various fitting connections.
The ferrule also secures on the tubing to prevent the tubing from ejecting from the fitting at specified pressures. For instance, HPLC ferrules are typically rated for pressures up to 6,000 PSI, and UHPLC ferrules are typically rated for pressures up to 20,000 PSI. In UHPLC systems, stainless steel tubing is often used to accommodate the high pressures. The ferrules are also typically made of stainless steel to properly seal against the tubing and to prevent the tubing from ejecting at the high pressures. When the ferrule is forced into the female cone-shaped chamber of the fitting, the ferrule swages down onto the tubing to prevent the tubing from ejecting from the fitting. However, with the ferrule being made of stainless steel, the ferrule swages onto the stainless steel tubing as a hard swage. As such, the position of the stainless steel ferrule cannot be readjusted on the tube, if, for instance, it is desired to use the tubing with a different fitting or component. Thus, if the stainless steel ferrule/tubing is reused in a fitting of a slightly different size, a “dead volume” is likely created between the end of the tube and the cylindrical chamber tube stop, or the ferrule cannot seat in the female cone-shaped chamber of the fitting, thereby causing the connection to leak.
In HPLC systems, a ferrule made of a softer material may be used such that a hard swage does not result. For instance, a ferrule made from Polyetheretherketone (PEEK) or another similar material may be used to seal the tubing within the fitting. The PEEK ferrule creates a soft swage on the tubing; and therefore, the position of the PEEK ferrule can be adjusted for use within different fittings. However, PEEK ferrules cannot withstand the extreme pressures of UHPLC systems.
Thus, it is desired to have a ferrule that can be re-used in various UHPLC fittings while maintaining a liquid-tight seal and preventing the tubing from ejecting at high pressures.
In certain HPLC and UHPLC systems, it is sometimes desirable to use an inert capillary tube that may include polymeric material or fused silica. For instance, the tubing may be comprised of polymeric-clad fused silica or fused silica with polymeric sleeves at the ends of the tubing. To be used within UHPLC systems, however, the tubing must be able to withstand extreme pressures in excess of 5,000 psi, often approaching 20,000 psi and beyond. In that regard, the UHPLC systems require fitting ferrule assemblies to secure tubing in place while making and maintaining a leak-tight seal.
As noted above, traditional stainless steel ferrules work on stainless steel tubing in a single-use fashion by swaging or crimping onto the tube to grip and hold the tube in place within the fitting, while at the same time forming a seal at the connection. If stainless steel ferrules (or similar ferrules) are used with fused silica tubing, it is desirable to sleeve or clad the fused silica tubing with an outer layer of polymer, such as PEEK. The PEEK outer sleeve provides a cushion between the stainless steel ferrule and the fused silica tube to prevent fracturing of the fused silica.
A polymeric ferrule, such as a PEEK ferrule, creates a soft swage on the tubing. Therefore, a PEEK ferrule or similar ferrule does not fracture or deform the more delicate tubing, such as polymeric-clad fused silica or fused silica with polymeric sleeves at the ends of the tubing. Moreover, the tubing can be repositioned with respect to the PEEK ferrule, and the position of the PEEK ferrule can be adjusted for use within different fittings.
However, PEEK ferrules in use with these tubings cannot withstand the extreme pressures of UHPLC systems. Higher pressures are difficult to achieve with polymer ferrules due to the limited gripping ability of the ferrules on the outer polymer layer of the tubing. At high pressures, the capillary tube ejects, resulting in a system failure. In addition, the polymeric ferrule may creep on the tubing over time due to its limited gripping ability or due to elevated temperatures, thereby necessitating retightening within the fitting to reinstate the grip of the ferrule on the capillary tube.
A wrench or similar tool may be used to tighten the fitting assembly and further compress the ferrule within the fitting assembly and onto the tubing, thereby increasing the pressure capabilities of PEEK or similar ferrules. However, by using a wrench to tighten the fitting, as opposed to tightening the fitting by hand, the tubing is often forced too tightly into the fitting port. For instance, referring to FIG. 1, tubing 34 would be driven into the cylindrical chamber 32 and may abut up against the tube stop 30. If a more delicate material is used for the tubing, such as silica, the tubing may fracture due to the over tightening.
Thus, there is additionally a need for a fitting and/or ferrule assembly that grips and seals onto various types of tubes (such as stainless steel, polymer-clad fused silica, polymer-sleeved fused silica, etc.) used in HPLC and UHPLC systems without breaking or deforming the tube.